The Io torus produces ultraviolet emissions diagnostic of plasma conditions. We revisit data sets obtained by the Voyager 1, Galileo, and Cassini missions at Jupiter. With the latest version (8.0) of the CHIANTI atomic database we analyze UV spectra to determine ion composition. We compare ion composition obtained from observations from these three missions with a theoretical model of the physical chemistry of the torus by Delamere et al. (2005). We find ion abundances from the Voyager data similar to the Cassini epoch, consistent with the dissociation and ionization of SO2, but with a slightly higher average ionization state for sulfur, consistent with the higher electron temperature measured by Voyager. This reanalysis of the Voyager data produces a much lower oxygen:sulfur ratio than earlier analysis by Shemansky (1988), which was also reported by Bagenal (1994). We derive fractional ion compositions in the center of the torus to be S+/Ne ~ 5%, S++/Ne ~ 20%, S+++/Ne ~ 5%, O+/Ne ~ 20%, O++/Ne ~ 3%, and Σ(On+)/Σ(Sn+) ~ 0.8, leaving about 10–15% of the charge as protons. The radial profile of ion composition indicates a slightly higher average ionization state, a modest loss of sulfur relative to oxygen, and Σ(On+)/Σ(Sn+) ~ 1.2 at about 8 RJ, beyond which the composition is basically frozen in. The Galileo observations of UV emissions from the torus suggest that the composition in June 1996 may have comprised a lower abundance of oxygen than usual, consistent with observations made at the same time by the EUVE satellite.

The satellite Io, which has volcanoes and is located at 5.9 RJ from the center of Jupiter, is a powerful plasma source in the magnetosphere. The heavy ions originating from Io form a torus‐like structure and emit radiation. The pickup energy and hot electrons are believed to power the Io plasma torus. Voyager data showed that a trace amount of hot electrons (at several hundreds of eV) exist in the torus. The origin of hot electrons, that is, plasma heating and/or transport mechanisms, have been mentioned in previous research. However, the contribution of each mechanism toward supplying hot electrons remains poorly understood. To address this issue, we explored the time variation and spatial structure of hot electrons by spectroscopic observations using the Hisaki satellite. In this study, the radial distributions of plasma densities and temperatures were derived from the emission line intensities in the extreme ultraviolet range of day of year (DOY) 331 in 2014 to DOY 134 in 2015, which includes the Io's volcanically active period. We found that hot electrons inside the torus began to increase particularly on the duskside ~40 days after the onset of volcanic activity. This result suggests that the mass increase in the torus with volcanic activity enhanced the plasma transport from the outside within a specific region or via a local heating process.

We have developed a spectral emission model that is a function of the plasma composition, electron temperature, and density in the Io plasma torus. The lines are excited by electron collisions and spontaneously decay resulting in UV emission that is diagnostic of the plasma conditions. In a previous study we used a single Maxwellian distribution to model the UV spectra obtained by the Cassini UVIS instrument in January 2001. We now try to determine the fraction of hot electrons using a double Maxwellian distribution where the core, thermal electron distribution is combined with a hot electron distribution, also assumed to be a Maxwellian. This spectral emission model does not well constrain the fraction of hot electrons, which can be seen by the χ 2 output. By using physical chemistry modeling of equilibrium conditions, we determine the fraction of hot electrons. Our physical chemistry model shows that in order to match the plasma composition and temperatures near the orbit of Io during the Cassini flyby of Jupiter we need the hot electrons to comprise <0.3% of the total electron density.